This invention relates to sheet handling apparatus for separating sheets from a stack and transporting, registering and deskewing individual separated sheets. In a particular application, it is directed to use of such sheet handling apparatus as an automatic document feeder in electrostatographic printing apparatus.
In an electrostatographic reproducing apparatus commonly in use today, a photoconductive insulating member is typically charged to uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or back ground areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the usual document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with developing powder referred to in the art as toner. Most development systems employ a developer material which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development the toner particles are attracted from the carrier particles by the charge pattern of the image areas in the photoconductive insulating area to form a powder image on the photoconductive area. This image may subsequently be transferred to a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure.
In order to increase the throughput and other reproducing capabilities it is common practice to use document handlers with automatic electrostatographic reproducing apparatus. The document handlers are used to separate individual documents from a stack of documents to be copied, move them on to the viewing platen where they are registered, held until the required number of copies have been made and then moved onto an output collection point to be followed by successive documents in the stack. Such an automatic document handler must not only move the document but must accurately register it in a predetermined copying position to assure production of a complete and visually acceptable copy. If, for example, the document is situated on the platen in a skewed or misaligned position, the copy will reflect the same skew or misalignment. Furthermore, it must also be capable of accepting a maximum range of paper weights or material weights efficiently as documents copied may have to vary from very heavy papers to very light papers such as those known as onion skin. In addition, it is important that the automatic document handler have the ability to readily accommodate manual documents such as books which necessitates that the viewing platen be cleared and easily accessible to the operator for such operations.
Typically document handlers that have been used in the prior art include those employing a wide friction belt to transport the documents across the viewing platen. These devices, however, have relatively limited latitude because the normal force that is required to ensure that the document can be driven across the platen glass is present across the whole document. That is the same normal force is applied at the leading edge, center and the trail edge of the document. This uniform normal force tends to inhibit deskewing of the document once its lead approaches the registration edge since the uniform normal force tends to inhibit the document from rotating laterally. Furthermore, since the friction drive force must always be sufficient to drive the document across the platen, unless that force is reduced it can buckle or damage lighter weight documents while driving them into the registration edge. In addition, these devices suffer from further difficulty in that they are relatively expensive to manufacture, which is a critical element today in the design, manufacture and marketing of automatic reproducing machines designed for low copy rate or low volume production. Most of the automatic document feeders that are used for the low volume segment use friction retard separation devices which are relatively unpredictable in that the exact period of time between separation and when the document is fully being transported is uncertain. Accordingly, these systems typically have a separator feed roll actuated by an expensive clutch together with clutched takeaway rolls and a sensor to determine when the document is well on a controlled path. Such a device is relatively expensive in that motor, clutches and bearings together with the sensor are all required in order to ensure controlled feeding of the document.